The present invention generally relates to friction stir welding tools and welding processes, more particularly, to a counter-rotating spindle for friction stir welding, to a continuous path friction stir welding process, and to a process for spot welding.
Welding may be the most common way of permanently joining metal parts. Friction stir welding (FSW) was introduced in late 1991 by the TWI Welding institute, U.K. (U.S. Pat. No. 5,460,317). In friction stir welding, a cylindrical, shouldered tool with a profiled pin is rotated and slowly plunged onto the work-pieces of sheet or plate material, which are lapped or butted together. The work-pieces have to be clamped rigidly onto a backing bar or engineered substructure in a manner that prevents the abutting joint faces from being forced apart. Frictional heat is typically generated between the wear resistant welding tool and the material of the work-pieces. This heat, along with the heat generated by the mechanical mixing process and the adiabatic heat within the material, causes the stirred materials to soften without reaching the melting point, allowing the traversing of the welding tool along the weld line. The plasticized material is transferred from the leading edge of the tool to the trailing edge of the tool pin and is forged by the intimate contact of the tool shoulder and the pin profile. The welding of the material is facilitated by severe plastic deformation in the solid state involving dynamic recrystallization of the base material. A solid phase bond between the two work-pieces is created. The friction stir welding process can be regarded as a solid phase keyhole welding technique since a hole to accommodate the pin of the tool is generated and then filled during the welding sequence. Since the friction stir welding process takes place in the solid phase below the melting point of the materials to be joined, materials that are difficult to weld using traditional fusion welding methods can now be joined, for example, 2000 and 700 aluminum alloys. Other advantages include, for example, excellent mechanical properties of the weld, usage of a non-consumable tool, and elimination of the need for consumable welding products. The described friction stir welding process using a tool with a fixed pin length has the disadvantages of only being able to join work-pieces having the same thickness, and of leaving a keyhole in the work-piece when the pin is refracted at the end of the weld line. This hole must be covered, for example, with a rivet in order to preserve the integrity of the weld. Furthermore, the friction stir welding process is usually used to produce straight-line continuous path welds. Due to the forming of the keyhole when retracting the tool, friction stir welding is typically not used for spot welding. The friction stir welding process can be used for the manufacture of many joint geometries, such as butt welds, overlap welds, T-sections, fillet, and corner welds. For each of these joint geometries specific tool designs are required.
An auto-adjustable pin tool for friction stir welding was developed by engineers at Marshall Space Flight Center of the National Aeronautics and Space Administration (NASA) (U.S. Pat. No. 5,893,507). The retractable pin tool (RPT) uses a computer-controlled motor to incrementally retract the pin into the shoulder of the tool away from the work-piece at the end of the weld preventing keyholes. The design of the tool allows the pin angle and length to automatically adjust to changes in material thickness and results in a smooth hole closure at the end of the weld. Current retractable pin tools utilize two separate parts, a shoulder and a pin. The pin is positioned within the shoulder. When welding the rotating pin enters the work-pieces to be welded and stirs the material while the shoulder impacts the surface of the work-pieces. Since there is a tolerance between the shoulder and the pin, hot material may be able to migrate up the shaft between shoulder and pin during the welding process. Once the migrated material cools it may temporary and locally weld the pin to the shoulder. This galling effect may damage the surfaces of the pin and/or the shoulder. In all known retractable pin tool applications the pin and shoulder rotate at the same speed and in the same direction. In some applications, depending on the material to be welded, overheating or even melting along the work-piece surface close to the shoulder may occur that may be caused by the rotation of the shoulder. This can lead to undesirable fusion related defects of the work-piece surface.
As can be seen, there is a need for a friction stir welding tool that may be used for a variety of joint geometries, that eliminates the galling effect between the shoulder and the pin during the welding process, and that eliminates overheating of the work-piece surface close to the shoulder of the tool. Furthermore, there is a need for a friction stir welding tool that may be used to reduce the surface indent during the friction stir spot welding process. Moreover, there is a need for a friction stir welding process that produces welds with improved smoothness over the entire length and with improved durability of the joints.
There has, therefore, arisen a need to provide a versatile friction stir welding spindle. There has further arisen a need to provide a stir friction welding spindle that enables an improved mixing of the materials to be welded. There has still further arisen a need to provide a process for friction stir welding producing continuous path welds that have optimized mechanical properties and are smooth over the entire length. There has still further arisen a need to improve the friction stir spot welding process by reducing material loss, by reducing surface indent, and by eliminating weld nugget voids.